The present invention relates to a multilayer ceramic substrate having an interlayered capacitor.
A plurality of semiconductor chips are mounted directly on a multilayer ceramic substrate. Each semiconductor chip may be an IC or an LSI. A capacitor can be formed within the substrate by sandwiching an inner ceramic sheet between a pair of conductor layers. In such an integrated capacitor substrate, conductor pads are formed on the top sheet and wirings are formed within the substrate to connect the capacitor and the pads.
The capacitor substrate is manufactured by a green sheet laminating technique. According to this technique, green sheets are successively laminated and then are sintered or fired. The wirings are printed on each green sheet before the successive laminations. In a conventional green sheet laminating technique, alumina is used as the green sheet. The laminated green sheets must, therefore, be sintered at a sintering or firing temperature of 1,500.degree. C. or higher. In view of the high sintering temperature, the wirings must be made of molybdenum, tungsten, or a like conductor material having a high melting point.
For such a high melting point conductor material, sintering must be carried out in a reducing atmosphere as, for example, in a hydrogen furnace. Due to the necessity of a high sintering temperature and a reduced atmosphere, the installation for manufacture is large scale, expensive, and objectionable from the viewpoint of energy saving.
The high melting point conductor material has a poor electrical conductivity. Each conductor for the wirings must, therefore, be relatively thick. This results in larger dimensions for the wirings.
Moreover, since all ceramic sheets are made of alumina, the dielectric constant is very low, say, nine or so, and a capacity of only about 3 pF/mm.sup.2 can be obtained.
On the other hand, when a material having a high dielectric constant, such as steatite, forsterite, berium titanate, titanium dioxide, etc., can be used for the ceramic sheet instead of alumina, the mechanical strength of the substrate is low. Although, it can be utilized satisfactorily as a chip component, it is not acceptable for use as a circuit substrate.
It is mandatory, in general, for a multilayer substrate to have sufficient mechanical strength to support semiconductor chips and supporting structures mounted thereon. It is preferable that a multilayer substrate should have a flexural strength which is not less than 1,500 Kg/cm.sup.2.